Seal types

The purpose of a seal is to retain lubricant and prevent any contaminants from entering into a controlled environment. To be effective, a seal should exhibit the following main characteristics:

flexible enough to compensate for any surface irregularities

strong enough to withstand operating pressures

able to accommodate a wide range of operating temperatures

resistant to common chemicals

operate with lowest possible friction, frictional heat and wear

There are several basic seal types:

static seals

dynamic seals

non-contact seals

bellows and membranes

Seals that make contact with stationary surfaces are called static seals. Their effectiveness depends on the radial or axial deformation of their cross section when installed. Gaskets (fig. 1) and O-rings (fig. 2) are typical examples of static seals.

Seals in contact with sliding surfaces are called dynamic seals and are used to seal passages between machine components that move relative to each other either linearly or to retain lubricant, exclude contaminants, separate different media and withstand differential pressures. There are various types of dynamic seals, including packing or piston rings, which are used for linear or oscillating movements. However, the most common seal is the radial shaft seal (fig. 3), which makes contact with both a stationary and rotating component.

Non-contact radial shaft seals form a narrow gap between the stationary seal lip and rotating component. The gap can be arranged axially, radially or in combination. Non-contact seals, which range from simple gap-type seals to multi-stage labyrinths (fig. 4), generate almost no friction and, therefore, do not wear.